We describe the mixed-methods (qualitative and quantitative) development and preliminary validation of the Patient Assessment for Low Back Pain–Symptoms (PAL-S), a patient-reported outcome measure for use in chronic low back pain (cLBP) clinical trials. Qualitative methods (concept elicitation and cognitive interviews) were used to identify and refine symptom concepts and quantitative methods (classical test theory and Rasch measurement theory) were used to evaluate item- and scale-level performance of the measure using an iterative approach. Patients with cLBP participated in concept elicitation interviews (N = 43), cognitive interviews (N = 38), and interview-based assessment of paper-to-electronic mode equivalence (N = 8). A web-based sample of patients with self-reported cLBP participated in quantitative studies to evaluate preliminary (N = 598) and revised (n = 401) drafts and a physician-diagnosed cohort of patients with cLBP (N = 45) participated in preliminary validation of the measure. The PAL-S contained 14 items describing symptoms (overall pain, sharp, prickling, sensitive, tender, radiating, shocking, shooting, burning, squeezing, muscle spasms, throbbing, aching, and stiffness). Item-level performance, scale structure, and scoring seemed to be appropriate. One-week test–retest reproducibility was acceptable (intraclass correlation coefficient 0.81 [95% confidence interval, 0.61-0.91]). Convergent validity was demonstrated with total score and MOS-36 Bodily Pain (Pearson correlation −0.79), Neuropathic Pain Symptom Inventory (0.73), Roland-Morris Disability Questionnaire (0.67), and MOS-36 Physical Functioning (−0.65). Individual item scores and total score discriminated between numeric rating scale tertile groups and painDETECT categories. Respondent interpretation of paper and electronic administration modes was equivalent. The PAL-S has demonstrated content validity and is potentially useful to assess treatment benefit in cLBP clinical trials.